PROJECT SUMMARY/ABSTRACT Like the two faces of Janus, antibodies can be a gateway to anti-microbial immunity or to autoantibody- associated disease. The recent successful use of anti-CD20 to treat patients with autoimmune dyscrasias has re-focused attention on the role of B cells as therapeutic targets. Mature B cells become antibody secreting plasma cells through a complex process that begins in the germinal center and involves alterations in multiple transcription factors. Work from this laboratory has identified a new player in B cell activation and differentiation, namely the novel gene, Faim. FAIM is unique;it is highly evolutionarily conserved, yet does not contain sequence homology, or structural homology, with any other protein. In B cells FAIM acts as a force multiplier. It boosts CD40 signaling by enhancing CD40L-stimulated increases in NF-[unreadable]B and IRF4, and, as a result thereof, it further reduces the CD40L-stimulated decline in BCL-6. As expected from the decline in BCL-6 (and its location in the germinal center) FAIM overexpression augments the plasma cell compartment in chimeric mice. FAIM expression is stimulated by IRF4 and so once triggered FAIM is involved in a "feed-forward" mechanism. The long term objective of this proposal is two- fold: to understand normal B cell biology focusing on how "resting" B cells become effectors, and to determine the points at which these processes go awry resulting in autoantibody production and autoimmunity. The near term objective of this work is to define the role of FAIM in facilitating immunity and regulating autoimmunity, with the goal of identifying a new therapeutic target. The specific aims of this proposal are to: 1) conduct a careful molecular structure/function analysis to identify and characterize the unique FAIM effector motif;2) evaluate the influence of FAIM on the quality (affinity, idiotype, and autoreactivity) of antibody produced in a model immune response and elucidate the physiology of FAIM expression in the germinal center;and, 3) determine the role of FAIM in regulating autoimmune disease using the Sle1 and Sle1Sle3 models of autoimmunity. The results of this work are highly likely to provide completely new and fundamental information about how signaling in B cells is promoted, and about how plasma cell differentiation is regulated. Moreover, the recent finding by other investigators (unpublished) that SNPs proximal to, and within, the FAIM sequence are strongly associated with human lupus disease indicates that the mechanisms revealed by this study are highly likely to be relevant to understanding clinical autoimmunity and may provide a new target for therapeutic manipulation.